The present invention relates to a speaker damper, in particular to a speaker damper for use in a loudspeaker unit having an elliptical or rectangular shape.
There has been known a dynamic cone speaker unit in which electric signals may be converted into acoustic signals so as to produce sounds. The dynamic cone speaker unit has a conical vibrating diaphragm, the center of which is integrally connected with a voice coil bobbin winded by a voice coil. When electric current of sound signals is flowing through the voice coil, the voice coil (located in a magnetic gap) will be driven in a predetermined direction, causing the voice coil bobbin and the vibrating diaphragm to vibrate integrally, hereby producing acoustic signals.
The voice coil bobbin secured at the center of the vibrating diaphragm is also supported by a damper. In operation, the voice coil bobbin is caused to vibrate in the above predetermined driving direction together with a circumferential edge member supporting the outer circumference of the vibrating diaphragm, but not liable to vibrate in a direction orthogonal to the above predetermined driving direction.
The damper is constructed and installed in such a manner that when the voice coil is driven along the magnetic gap, the voice coil will not come into contact with a pole piece or a yoke (which are used to form a magnetic circuit). Further, the damper also serves to keep the vibrating diaphragm and the voice coil bobbin in positions when the voice coil is not driven.
For this reason, the damper is required to have the same characteristics as the above edge member, to ensure a sufficient amplitude for the vibration of the diaphragm. As a result, such a damper should be made of a material having a suitable softness and made into a shape not liable to roll or move in a lateral direction.
In fact, there have been several kinds of speaker dampers such as a circular damper and a butterfly damper.
A circular damper is formed by impregnating a piece of circular cloth with a resin, heating the resin-impregnated cloth to dry the same, forming a plurality of corrugations which are concentric circles in the resin-coated cloth, followed by formation of a circular hole for holding a voice coil bobbin in the center of the damper.
A butterfly damper is made by punching a thin bakelite plate or a thin metal plate. Such a butterfly damper comprises an inner frame for holding a voice coil bobbin, a plurality of flexibly movable supporting arms connected with the inner frame, an outer frame connected with the flexibly movable supporting arms for supporting the damper on to a frame in a speaker unit.
However, if a speaker unit is to be installed in a motor vehicle or a thin type television set, such a speaker unit should also be thin in its total thickness.
One example of a thin type speaker unit is an elliptical speaker unit having an elliptical vibrating diaphragm. If a circular damper as described above is used in such a thin type speaker unit, the outer diameter of the circular damper is restricted by the size of an elliptical frame in its shorter axis direction. Consequently, it is difficult to obtain a sufficient compliance, resulting in a problem that it is impossible to ensure a low sound characteristic which is necessary for producing a desired lowest resonance frequency f.sub.o and desired vibration amplitude.
On the other hand, a butterfly damper may have an elliptical shape and a structure as shown in FIG. 2. As illustrated in FIG. 2, the butterfly damper has an outer frame 104 arranged in the longitudinal direction of an elliptical speaker unit. The butterfly damper has a circular inner frame 103 in the center thereof for holding a voice coil bobbin, a plurality of flexibly movable supporting arms 102 integrally formed with the circular inner frame 103 and the outer elliptical frame 104. The flexibly movable supporting arms 102 are all arranged in the longitudinal direction of the elliptical damper. In this manner, it is possible to ensure a desired lowest resonance frequency f.sub.o and desired vibration amplitude. However, since the flexible supporting arms 102 are caused to provide support action in the longitudinal direction to support the voice coil bobbin (not shown) held on the circular inner frame 103, it is difficult to ensure that the force supporting the voice coil bobbin is uniform surrounding the whole circumference of the bobbin. When the voice coil is driven, the voice coil bobbin and the vibrating diaphragm are liable to roll, resulting a problem that the voice coil undesirably gets into contact with a pole piece and/or a yoke (which are used to form a magnetic circuit), hence undesirably producing an abnormal sound and/or damaging the voice coil itself.